1. Field of the invention
The present invention relates to a rear wheel steering control system for an automotive vehicle, more specifically to a steering control for rear wheels of a four wheel steering vehicle.
2. Description of the prior art
In a field of automotive vehicles, there has been known a four wheel steering system in which rear wheels are steered as well as front wheels thereof. One of the four wheel steering systems is a mechanical system in which a front wheel steering mechanism is functionally and mechanically connected with a rear wheel steering mechanism. The other is known as an electrical steering system in which the rear wheel steering mechanism is provided with a drive motor for driving the rear steering mechanism as shown in Japanese Utility Model public disclosure No. 62-25277 laid open to the public on Feb. 16, 1987. U.S. Pat. No. 4,645,025 discloses another electrical rear wheel steering system with an electric motor for driving the rear wheel steering system. The electrical rear wheel steering system disclosed in the Japanese publication is provided with a speed reduction mechanism between the motor and the rear wheel steering mechanism so as to reduce a capacity of the motor.
In a steering control, it is desirable that the rear wheels are prevented from steering under a certain condition such as a constant vehicle speed and a constant steering angle for steering wheel. For this purpose, so called servo lock system has been known in which a power is supplied for a servo motor for driving the rear steering mechanism so as to maintain a constant rotative position thereof and thereby to keep a constant steering angle of the rear wheels. The Japanese publication discloses a motor brake device for locking the motor as the steering lock system so that the rear wheel steering mechanism is locked to maintain a steering angle of the rear wheels at a constant value when the motor brake device is actuated. The motor brake device is actuated to lock the rear wheel steering mechanism when a vehicle speed and the steering angle of a steering wheel are constant. This steering lock system of the rear wheel steering system employing the above motor brake device is advantageous in that an electric power consumption can be remarkably reduced in comparison with the servo lock system. As a result, a durability of the motor can be improved.
However, it has been found that such steering lock systems are not responsive in a control of the steering angle of the rear wheels under a certain vehicle operating condition, because the motor cannot be actuated to change the steering angle of the rear wheels until the steering lock systems are released even when a steering angle control of the rear wheels is desired. Thus, the conventional steering lock system cannot satisfy a request for a quick responsiveness in the rear wheel steering control.